This paper presents an Fuzzy Neural Network model for inflow forecast for the Sobradinho Hydroelectric power plant, part of the Chesf (Companhia Hidrelétrica do São Francisco-Brazil) system. The model was implemented to forecast monthly average inflow on a one-step-ahead basis. The Fuzzy Neural Network model is shown to provide better representation of the monthly average water inflow forecasting, than the models based on Box-Jenkins method, currently in use on the Brazilian Electrical Sector.